Process for forming a phosphate conversion coating on metal

Abstract

A metal surface on which a phosphate conversion coating is to be formed and which has been surface conditioned by contact with a liquid surface conditioner composition that contains dispersed fine particles of solid phosphate of at least one divalent or trivalent cations type and an adhesion promoting agent. After such conditioning, a very high quality conversion coating can be formed on the surface by contact with a nickel-free liquid phosphating composition that contains at least acid, zinc cations, and phosphate anions and optionally and preferably also contains other materials.

What is claimed is:  
     
       1. A process for forming a phosphate conversion coating on a metal substrate surface, said process comprising the following operations: 
       (I) contacting the metal substrate surface with an aqueous liquid surface conditioning composition that comprises water and the following components:  
       (I.A) dispersed solid phosphate particles that:  
       (i) have a diameter no greater than 5 μm; and  
       (ii) comprise at least one substance selected from the group consisting of phosphates that contain at least one type of divalent or trivalent metal cations; and  
       (I.B) as adhesion-promoting component, at least one selection from the group consisting of the following subgroups:  
       (1) monosaccharides, polysaccharides, and derivatives thereof;  
       (2) phosphorus containing solutes selected from the group consisting of orthophosphoric acid, condensed phosphoric acids, and organophosphonic acid compounds;  
       (3) water-soluble polymers that are homopolymers or copolymers of vinyl acetate and derivatives of these homopolymers and copolymers; and  
       (4) copolymers and polymers afforded by the polymerization of:  
       (a) at least one selection from:  
       monomers, exclusive of vinyl acetate, that conform to general chemical formula:                    
        where R 1 =H or CH 3  and R 2 =H, C 1  to C 5  alkyl, or C 1  to C 5  hydroxyalkyl; and  
       other α, β-unsaturated carboxylic acid monomers; and, optionally,  
       (b) not more than 50% by weight of monomers that are not vinyl acetate and are not within the description of part (a) immediately above but are copolymerizable with said monomers that are within the description of said part (a); and  
       (II) contacting the metal substrate surface as conditioned in operation (I) as described above with a nickel-free phosphate conversion treatment bath that comprises water and the following amounts of the following components:  
       (II.A) from 0.5 to 5 g/l of zinc cations;  
       (II.B) from 5 to 30 g/l of phosphate ions; and  
       (II.C) a component of conversion accelerator.  
     
     
       2. A process according to  claim 1 , wherein the phosphate conversion treatment bath also contains from 0.1 to 3.0 g/l of at least one type of ions selected from the group consisting of magnesium ions, cobalt ions, manganese ions, calcium ions, tungstate ions, and strontium ions. 
     
     
       3. A process according to  claim 2 , wherein the concentration of component (1.A) is from 0.001 to 30 g/l and the concentration of component (1.B) is form 1 to 2,000 ppm. 
     
     
       4. A process according to  claim 1 , wherein the concentration of component (1.A) is from 0.001 to 30 g/l and the concentration of component (1.B) is from 1 to 2,000 ppm. 
     
     
       5. The process of  claim 1  wherein the divalent or trivalent metal cations are selected from the group consisting of Zn, Fe, Mn, Co, Ca, Mg and Al. 
     
     
       6. The process of  claim 1  wherein the aqueous liquid surface conditioning composition has a concentration of dispersed solid phosphate particles of from 0.001 to 30 g/l. 
     
     
       7. The process of  claim 1  wherein the aqueous liquid surface conditioning composition has a concentration of adhesion-promoting component of from 1 to 2000 ppm. 
     
     
       8. The process of  claim 1  wherein the aqueous liquid surface conditioning composition is additionally comprised of an alkali metal or ammonium salt selected from the group consisting of orthophosphate salts, metaphosphate salts, orthosilicate salts, metasilicate salts, carbonate salts, bicarbonate salts, nitrate salts, nitrite salts, sulfate salts, borate salts, organic acid salts and mixtures thereof. 
     
     
       9. The process of  claim 1  wherein the nickel-free phosphate conversion treatment bath is additionally comprised of an etchant selected from the group consisting of fluoride ions, complex fluoride ions, and mixtures thereof. 
     
     
       10. The process of  claim 1  wherein the metal substrate is selected from the group consisting of steel sheets, zinc-plated steel sheets, zinc alloy-plated steel sheets, magnesium alloys, and aluminum alloys.